The present invention relates to the magnetic resonance imaging and spectroscopy arts. It finds particular application in conjunction with steady-state magnetic resonance imaging techniques and will be described with particular reference thereto.
Heretofore, numerous steady-state magnetic resonance imaging techniques have been developed. Steady-state imaging techniques have very short repeat times. For human tissue subjects, the T1 and T2 relaxation times commonly exceed the repetition time. That is, the component of the transverse magnetization created by an earlier RF pulse is still present when the next RF pulse is applied. Thus, at any point in time during the sequence, there is cumulative transverse coherent signal which is the superposition of all decaying components remaining from earlier repetitions. The longer the T2 relaxation time relative to the repetition time, the more heavily T2 weighted the cumulative coherent signal becomes.
One technique for spoiling transverse coherences built-up between successive repetition intervals of a steady-state sequence is the application of spoiler gradients. See Crawley, et al., "Elimination of Transverse Coherences in FLASH MRI", Mag. Res. in Medicine, Vol. 8, pp. 248-260, 1988. In the Crawley technique, spoiler gradients are applied whose amplitudes change linearly with the phase encode step. The effect of such spoilers at a given location along the gradient is equivalent to the effect on the entire field of view of an increased phase shift applied to the radio frequency pulse. An appropriate RF phase shift scheme should spoil residual magnetization components in steady-state imaging.
For two-dimensional and volumetric imaging, the steady-state imaging sequence is typically preceded by a presaturation RF pulse. In many techniques, the RF pulse is applied concurrently with the slice select gradient. The presaturation pulse and slice select gradient saturate selected regions of the volume to eliminate contributions therefrom. Spoiler gradients along each of the three orthogonal gradient axes are applied between the presaturation pulse and the steady-state imaging sequence. In some techniques, the phase of the presaturation RF pulse is reversed in alternate applications. One of the drawbacks to this technique is that the duration of the spoiler gradient pulses add a significant duration to the very short repetition times of steady-state sequences.
Steady-state techniques are also used in sequences that require spectral saturation. In a spectral saturation sequence, a narrowed RF pulse is tailored such that selected frequencies are irradiated so that only a selected spectral band(s) is saturated, e.g. the methyl and methylene resonances of fat are saturated to suppress the response from fat.
The present invention provides a new and improved imaging technique which overcomes the above-referenced problems and others.